There is no requirement in the law of the land to 'register' a business. However, if you would rather acquiesce to statutory legislation and hence tax laws, you should register as self-employed with HMRC if operating as a sole-trader or partnership. If you have significant turnover then a ltd company may offer financial advantages.

First up what you pay is not relevant, its what they will pay that counts, and then if this is enough margin to make a good return or not.

I would talk to retailers about their mark-ups and what they want to sell the product for to get a range of prices you want to sell to them at. I say range because it should be all about the haggle. Expect to get knocked down, and significantly for larger retailers. Different items have different margins, but 2-3 times is common so you might be able to get the same yourself.

If you are selling "stuff" in any volume then you may well benefit from being VAT registered as the shops wont care about paying you VAT, and you will be able to recover it on your costs, meaning 20% lower costs for you.

We looked into this recently and around 40% seems to be the markups shops want to put on our products. Again, we found it changes from shop to shop and how much the buyer thinks they can make back and how much they want it in their shop